Several drugs with certain structural similarities (tricyclic ring system with dialkylaminoalkyl side chains) to tilorone, a potent interferon inducer, were screened for antiviral activity in vivo. Two acridine drugs, Acranil and quinacrine, were found to be effective, the former being almost as protective as tilorone and the latter less so. Both agents induced an interferon-like substance which could be detected in the serum of treated mice. The concentration of the inhibitory factor in the serum was highest after exposure to tilorone, followed in turn by Acranil and quinacrine, based on the administration of equal weights of drugs. Both tilorone and Acranil induced lower levels of circulating interferon-like substance in Balb/c mice than in other strains of mice. The serum factor induced by Acranil was shown to be stable at pH 2.

Acute X-irradiation with 350 R increased the synthesis of interferons induced by poly I : C and endotoxin in individual sera of BALB/c mice. Fractional irradiation with 4.7 or 9.4 I~ daily up to total dose levels of 728, 977 and 1828 R in three different experiments had no influence on serum interferon levels induced by Semliki Forest Virus (SFV), poly I:C or endotoxin. The Newcastle Disease Virus (NDV) induced interferon synthesis decreased in comparison to the non-irradiated control at a total dose of 400 R and attained the control level on further elevation of the total dose. The anti-NDV antibody production fell to one quarter of the control value after exposure to a total dose of 1828 R.

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